1. Field of the Invention
The present invention relates to an image decoding apparatus for reading and decoding image data coded by the MPEG-1 or other coding system using DCT and recorded for example on a video CD or the like, an image decoding process, and an image reproduction apparatus for displaying the decoded image data on a display device.
2. Description of the Related Art
In the compression and coding of an image, orthogonal transformation is performed so as to reduce redundancy. In this, discrete cosine transformation (DCT) is made wide use of as one method of orthogonal transformation exhibiting the best coding efficiency for the JPEG (color stationary image band compression and coding system of the Joint Photographic Image Coding Experts Group), the MPEG (motion picture coding system of the Motion Picture Coding Experts Group), the H.261 (motion picture compression and coding system for television telephones and television conferences standardized by the ITU-T), and other various coding systems.
The DCT in the MPEG-1 is defined by equation 1. ##EQU1## where, x, y are coordinates in pixel space and are integers of 0 to 7,
u, v are coordinates (orders) in frequency space and are integers of 0 to 7, PA1 g(x,y) is the pixel value at the coordinate (x,y), and PA1 f(u,v) is a coefficient (amplitude) of the spatial frequency (u,v).
The original image for transformation is divided into macro blocks of 8.times.8 pixels, then each macro block is subjected to DCT to obtain coefficients for each of the combinations of the horizontal 0th to 7th and vertical 0th to 7th spatial frequencies.
In the MPEG-1, the DCT coefficient f is quantized, subjected to variable length coding, and converted to one-dimensional information in order from the low frequency component, whereby an MPEG-1 bit stream is generated.
The decoding apparatus for decoding image data coded by such MPEG will be explained next referring to FIG. 1.
FIG. 1 is a block diagram of the configuration of a standard MPEG decoding apparatus 30.
The MPEG coded data input to a decoding apparatus 30 is subjected to variable length decoding at a variable length decoding unit (VLD) 31. The quantization information and the quantized DCT coefficients are input to an inverse quantization unit (Q.sup.-1) 32, while the motion vectors are input to a motion compensation prediction unit (MC) 35. The quantized DCT coefficient is subjected to inverse quantization at the inverse quantization unit 32 based on the quantization information and is further subjected to inverse DCT at an inverse DCT unit (DCT.sup.-1) to restore the spatial image data. This spatial image data is added with the motion compensation prediction data output from a motion compensation prediction unit 25 at an adder 34 to generate the original image data. When this image data is an I-picture or P-picture, it is stored in a frame memory (FRM) 36. Further, the motion compensation prediction unit 35 generates motion compensation prediction data based on the motion vector input from a variable length decoding unit 31 based on the image stored in the frame memory 36 and outputs it to the adder 34.
Due to these advances in the technology for compression and coding of image data, it has become possible to record long hours of motion picture data on a disk or other recording medium. As an example of use of the MPEG-1, a video CD etc. have been commercialized.
Further, due to these advances in image processing technology, to enable selection of any image data program, the screen may be divided for example into four or nine sections and a plurality of index images be displayed there or the individual images may be compressed to enable two programs to be simultaneously watched and other various methods of displaying images may be used.
When processing such image data, however, there are sometimes restrictions on the bands of the image data to be processed. In such a case, one generally used method is to use a digital filter for computations among pixels to remove the high frequency component of the spatial frequencies of the image. However, a two-dimensional digital filter for performing processing on image data is comprised of a multiplier, an adder, a plurality of registers, a ROM, delay elements, etc. and therefore there is the problem of a larger circuit size. In particular, in a decoding apparatus of the MPEG etc., since the circuit becomes larger in size, there is a demand for making the circuit simpler. Therefore, further use of digital filters cannot be said to be suitable. Further, the above processing of image data is not preferable in terms of the processing time since high speed processing is sought in many cases.
As cases where limitation of the band of the image data is necessary in this way, mention may be made of the case, explained previously, when compressing image data and displaying it on a screen.
For example, when a compression rate of 1/K (K is an integer) is sufficient, one row is extracted for every K number of rows of the original image and one column for every K number of columns so as to comprise a compressed image. This method is extremely simple in processing and can be realized at a high speed with a simple circuit.
In this method, however, the spatial frequency of the image generated expands into the high frequency side and moire-like noise occurs in the compressed image.
For example, in the case of generating a 1/2 compressed image such as shown in FIG. 2B by extracting the image for every other column from the image with two columns of white pixels and black pixels mixed alternately as shown in FIG. 2A, the white and black repetitive cycle in the image of FIG. 2A becomes 4 pixels, but is 2 pixels in the image of FIG. 2B. By compressing in 1/2 in the horizontal direction, the horizontal spatial frequency of the image becomes double.
That is, when the image is compressed without limiting the band of the spatial frequency, the relationship between the maximum spatial frequency fmax-s of the image compressed by the compression rate k (1/K) and the maximum spatial frequency fmax-o of the original image becomes the relationship shown in equation 2 and the band of the compressed image enlarges considerably to the high frequency side: EQU fmax-s=fmax-o/k (2)
Further, when the spatial frequency of the compressed image exceeds the spatial frequency displayable by the display device, noise is caused in the image.
Accordingly, in such a case, a demand has arisen to cause sufficient attenuation of the frequency component exceeding the limit of the display device.